1. Field of the Invention
The present invention generally relates to steam turbines. More specifically, the present invention relates to an apparatus and method for reducing relative motion between the blades of the turbine and the rotor, especially during turning gear operation. The present invention has particular application to steam turbines of the type employing "axial entry" blades, but is not limited thereto.
2. Description of the Prior Art
Steam turbines of the type employing "axial entry" blades comprise a rotor having a plurality of generally fir tree shaped generally axially extending grooves, with the blades circularly disposed therearound. Each blade has a generally fir tree-shaped root at a proximal end thereof for registration with one of the grooves and a shroud integral with the blade at a distal end thereof. During normal operation, under load, centrifigal force holds the blade roots tight in the grooves with which they register. During turning gear operation however, the centrifigal force is insufficient to hold the blade roots tight in their grooves and hence the blades "flop", i.e., the blade roots rock circumferentially and axially in the grooves. This relative motion between the blade root and the rotor grooves may cause fretting of the root.
Blade "flop" may aggravate other problems. For example, "axial entry, integral shroud" blades are often used in the first rows of some turbines because they are more reliable than riveted shrouds. Usually, the shrouds are tightly butted, but in certain rows a small gap is intentionally provided between adjacent shrouds to allow for thermal expansion. Faces of the shrouds may wear from snubbing that occurs as a result of blade "flop.
It is known that certain steam turbines manufactured by Brown Boveri Corporation incorporate a means intended to solve this problem. That means comprises coil springs disposed in a plurality of radially oriented holes in the bottom of each root. It is believed that the intent of the coil spring is to urge the root radially outward against the edges of the goove to prevent relative motion between the root and groove. This solution, however, is considered undesirable because it is mechanically complicated and further because the radially oriented holes weaken the root. Further, the amount of force that can be exerted by the coil springs is limited by the size of the hole that can be machined in the root, and also by the size of the spring.
The assignee of the present application has implemented two other prior art means for solving this problem. The first is to drive shims between the bottom-most portion of each root and the bottom of the groove with which that root registers. The second is to cement each root in its respective groove with an adhesive such as Loctite.RTM.. The problem with these solutions is that disassembly of the turbine is made difficult and time consuming.
It is therefore desirable to provide an apparatus and method for reducing blade "flop" which is simple, inexpensive, reliable and easy to implement.